Good Luck. I have faced this my entire tech life. There are lots of business using OpenSource products and I've made all the arguments. It's often about the back room business agreements "business partners" then any tech. For example, Hertz Car Rental buys IBM and Cisco because they use Hertz cars almost exclusively. You can't fight this.
Garner say 85 percent of companies are already using open source. In this day in age, if you companies is not using OS, they are a dinosaur.